The present invention relates to a process for the production of highly active, dispersed, low apparent density (L.A.D.) aluminum hydrate, to shaped components made of this highly active, dispersed low apparent density aluminum oxide which has a specific surface area of at least 200 m.sup.2 /g and features adequate resistance to fracture and wear, and relates too to processes for manufacturing the shaped components.
Several processes for manufacturing shaped components out of active aluminum oxides are known. Common to all these processes for manufacturing active aluminum oxide in granular form is that either ground alumina try-hydrate is transformed with the aid of binders into shaped components, which are then activated by heating under conditions of incomplete removal of water of constitution, or first an active aluminum oxide is prepared in powder form by heating alumina hydrate, then the said powder is ground and granulated under water.
According to the German patent publication No. DE-OS 22 27 804 active aluminum hydroxide granulate materials prepared by the latter method exhibit a specific surface area by BET of 300-450 m/g, a strength at fracture of 15-30 kg and a total pore volume of 40-50 cm.sup.3 /100 g when prepared by roll granulation. The above mentioned pore volume is much too small for use as catalyst or catalyst support impregnated with catalytic active substances. Material transport in a voluminous body is of course much faster the greater the porosity: the "catalytic activity" is a function of the pore volume. There has therefore been no scarcity of proposals for ways of achieving the largest possible pore volume in active alumina granulate materials. Apart from the expense incurred by the process, one is forced to accept a much reduced fracture strength, as shown in example No. 2 of patent publication No. DE-OS 24 39 543, issued as supplementary to No. DE-OS 22 27 804.
In many processes organic substances are added to the granulate then later burnt off so that pores are produced.
The already mentioned German patent publication No. DE-OS 24 39 543 proposes adding to the finely ground active aluminum oxide powder, prior to the granulation stage, finely ground residue-free, water-soluble salts and/or non-ionogenic substances which do not react with aluminum oxide. The fractions of the mixtures are 30-90 wt.-% aluminum oxide and 10-70% additive. The mixture is granulated under water, the granulate aged for at least 5 hours at 50.degree.-100.degree. C. and then the water-soluble additive washed out. Finally the granulate is dried and activated in the normal manner.
Both processes for manufacturing porous aluminum oxide using additives feature considerable disadvantages. Adding organic substances causes a reduction in the specific surface area because of the high temperatures required to burn off the organic fraction. According to the German patent publication No. 24 39 543 there are considerable effluent problems. Compared with shaped components which are made without additives there is a definite penalty in terms of strength.
German patent publication No. DE-OS 32 43 193 reveals a hydrated aluminum oxide, which can be used as a catalyst, contains essentially pseudo-boehmite, less than 400 ppm Na.sub.2 O, less than 3 wt.-% sulphate, and has a peptisation index of less then 60 minutes and a free surface area of 200-400 m.sup.2 /g. Its manufacture is such that an aluminum sulphate solution and a sodium aluminate solution are added simultaneously to a water-containing device. This is conducted in such a manner that a constant pH of &lt;5 is maintained during the precipitation process. After this the pH is shifted to the alkaline side by further addition of sodium aluminate liquor. The precipitated pseudo boehmite is aged, filtered off, washed and dried.
All commercially available, shaped active aluminum oxide components have the disadvantage that, if they have to exhibit adequate resistance to fracture and wear, they have a maximum pore volume of 0.7 cm.sup.3 /g. Furthermore, expensive wet chemical methods are necessary for their manufacture, aluminumhydroxide first being dissolved and then re-precipitated by addition of ammonia or acids.